CA1083848A

CA1083848A - Sensitive j-tube and applications thereof

Info

Publication number: CA1083848A
Application number: CA283,344A
Authority: CA
Inventors: David G. Nolte
Original assignee: Texaco Development Corp
Current assignee: Texaco Development Corp
Priority date: 1976-08-09
Filing date: 1977-07-22
Publication date: 1980-08-19
Also published as: DE2734491A1; JPS5919288B2; GB1568865A; JPS5326162A; DE2734491C2

Abstract

METHOD AND APPARATUS FOR MEASURING
A CHANGE IN SURFACE LEVEL OF A FLUID
(D#74,619-F) ABSTRACT
A sensitive J-tube employs an indicating liquid in the U-tube portion. Such indicating liquid is immiscible with the liquid thereabove. The longer leg of the J-tube has a larger cross-section area portion which may contain the above liquid when the J-tube has been immersed therein.
There is valve means in order to connect both legs of the U-tube portion at locations that are above the indicating liquid, into communication with the above liquid the level of which is to be measured, and the valve means may isolate the longer leg of the J-tube during a level change measure-ment.

-I-

Description

108384~3 BACKGROUND OF T~ TENTION - -.
Field of the Invention This invention concerns a sensitive J-tube instrument and a particular application thereof. More specifically it concerns an application of the principles involved to provide a novel manometer that has increased sensitivity. Also, a particular application is that of measuring the change in liquid level of a tank of liquid over a predetermined period of time, which change may indicate the presence of a leak.

Description of the Prior Art The testing of underground tanks for leakage has always been a difficult problem. Heretofore, two known methods which have been used have had various draw-backs. A first method required the product to be removed from the tank and the vent lines to be sealed. Then the tank was pressurized with air to several pounds per square inch. The pressure would then be monitored over a given period of time for indications of a leak.
Obvious drawbacks of that arrangement included the fact that changes in pressure indicated would not necessary mean that the tank was leaking but it might just as well be in the connecting lines or dispensor from the tank. Furthermore, the removal of products from the tank was time consuming and costly, and the pressurizing o~ the tank risked damage thereto.
A second procedure which has been employed involved the fitting of the fill pipe of the tank with a four or five foot high standpipe, and then the tank would be filled with product to the top of the standpipe. Any lea~age would then ~e detected by observing the drop in level of the standpipe. This method had a bad tendency to stress the tank and could expand the ends of the tank. Also, since product had to be added to the tank for this test the tem-perature would not be stable, and consequently con-siderable variations would have to be taken into consideration to make an accurate determination.
Consequently, it is an object of this invention to provide an instrument and/or method for determining leaks in large product tanks, such as underground storage tanks for gasoline or the like.
It involves the principles of this invention which provides an amplification of the difference in surface levels of a pair of liquid columns that are connected by a U-tube.

SUM~ OF THE INVENTION
....

Briefly, the invention concerns a sensitive J-tube devise for measuring change in the surface level of a fluid. It comprises a relatively small inside diameter U-tube containing a predetermined quantity of a colored indicator-fluid that is immiscible with said fluid the surface level of which is to be measured.
The said indicator-fluid has a density which is approximately the same but heavier than said fluid being measured. The combination comprises a relatively large inside diameter tube connected to one leg of .

108384~3 said U-tube and forming therewith the taller leg of said J-tube. The said large tube i5 long enough to extend ~rom above the maximum surface level of said fluid being measured to below the minimum surface level thereof. It also comprises a three-way valve having three ports for connection to each of the legs of said U-tube and to the body of said fluid being measured, - and it comprises means for connecting said U-tube leg ports to said U-tube at a location that is above the maximum height of said indicator-fluid therein. The - said three-way valve having at least three positions one for connecting all three of said ports together and one for connecting only the shorter leg of said J-tube to the body of said fluid being measured and one for closing said U-tube leg ports. It also comprises a reservoir connected into the shorter leg of said J-tube at a location above the maximum expected height of said indicator fluid, in order to contain any overflow without lost of indicator fluid. And it comprises an elongated support for holding said J-tube device in a fixed position when a measurement is being taken. It al~o comprises an elongated handle for actuating said three-way valve from a location above the surface of said fluid being measured.
Again briefly, the invention concerns a method of magnifying the measurements of a change in surface level of a first fluid by employing a U-tube containing a second fluid which is immiscible with said first fluid. The method comprises the steps of connect-ing a relatively large cross-section area tube to one 1083~48 leg of said U-tube at a location that is higher than a predetermined maximum level of risc of said second fluid in one of the legs of said U-tube. It also comprises the steps of connecting both legs of said U-tube into communication with said first fluid at a location above said maximum level prior to the commencement of a measurement time period, and immersing said U-tube and said large area tube in said first fluid to equali~e the level of said first fluid inside of said large area tube with that outside, and equalize the levels of said second fluid in said U-tube legs. It also comprises isolating said one leg and the large cross-section area tube from said first fluid at said commencement of said measurement time period, .
and determining the difference in fluid levels of said second fluid in said U-tube legs at the termination of said measurement time period.
Once more briefly, the invention concerns a sensitive J-tube instrument for measuring change in the surface level of a fluid in an underground tank or the like, comprising a U-tube portion containing an indicator fluid therein, said indicator fluid being immiscible with said fluid the change in surface level of which is to be measured, said U-tube portion having a predetermined inside cross-section area, one leg of said U-tube portion being connected to a larger inside cross-sectional area tube to form the taller leg of said J-tube, said larger inside cross-sectional area tube having a length greater than the maximum difference in surface levels of said fluid being measured between the maximum and minimum surface levels during a measurement thereof, and valve means for connecting each of said U-tube portion legs with said fluid being measured and for isolating said taller leg, said valve means being connected above the tops of said indicator fluid.

Again briefly, the invention concerns a methodof measuring a change in surface level of a fluid, comprising the steps of taking a relatively small inside cross-sectional area U-tube having an indicator-fluid therein, connecting a larger inside cross-sectional area tube to one leg of said U-tube, immersing said tubes into said fluid with said larger ' -' -: ' ~ . ` ~: '', ' "': ' ' area tube extending above said surface level, connecting both legs of said U-tube with said fluid at a location on said tubes which is above the maximum height of said indicator-fluid therein, said connection being made while said tubes are immersed, disconnecting said one leg from said fluid to isolate said leg and said larger area tube from said 1uid at the beginning of a measuring time period, and determining the difference in height of the tops of said indicator-fluid levels in said U-tube at the end of said measuring time period whereby th0 change in surface level of said fluid is magnified.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects and benefits of the invention will be more fully set forth below in connection with the best mode contem-plated by the inventor of carrying out the invention, and in connection with which there are illustrations provided in the drawings, wherein:
Figure l is a schematic diagram illustrating a J-tube structure according to one embodiment of the invention;
Figure 2 is a schematic diagram like that of Figure 1 but showing the fluid level modified in accordance with changes in the level of a bulk fluid that is having its surface level change measured;
Figure 3 is a longitudinal elevation not according to scale, illustrating one embodiment of an instrument in accordance with the invention;
Figures 4 and 5 are schematic diagrams illustrating two operative positions of the three-way valve that is employed in the instrument according to Figure 3; and Figure 6 is a schematic cross-sectional diagram illustrating a sensitive manometer structure in accordance with the invention.

.

.: ~ . ,. .. . . ... . : : :

~V8384~

DESCRIP~ION OF THE PREFERRED EMBODIMENTS

It has been discovered that a change in the level of a body of liquid may be amplified by employing a J-tube instrument according to this invention. The basic principles are illustrated and will be explained in connection with the schematic showings of FIGS. 1 and 2. Both of these figures illustrate a body of liquid 11 that has a.surface 12, which may change in height, as indicated in FIG. 2. .
A change in the height of the surface 12 of liquid body 11 may be because of a leak in the tank (not shown), or other container for the liquid 11. However, such change would be slow and small in many instances.
And, particularly where an underground tank of gasoline or the like is concerned, the change in level is difficult to detect.
.It will be noted that the instrument includes a U-tube portion 15, which has a shorter leg 16 connected by the U-shaped bottom with a longer leg 17, on the other side. ..
Connected to the top of the leg 17 of the U-tube, there is a larger cross-sectional area tube 20 that is long enough to extend up above the maximum level of surface 12 of the liquid body 11.
There are two valves 23 and 24 that are connected as indicated, so that the top of leg 16 has valve 23 controlling the opening or closing of .the interior of this leg through the top, in regard to connection thereof with the body of fluid 11. The valve 24 is connected so as to similarly control a ~luid connection from the inside of the leg 17 of the U-tube lS to the body of 1uid 11. In this case there is a schematic indication of a pipe or other fluid conduit 27 from the upper portion of the U-tube leg 17, to one side of the valve 24. The other side of valve 24 is connected directly into the body of liquid 11, similarily as is the other side of the valve 23, which is connected to the top of the leg 16 of the U-tube lS.
In the U-tube 15 there is an indicator fluid 30, which is immiscible with the liquid 11. As will appear from the explanation which follows, the indicator liquid 30 may be slightly heavier than the liquid 11.
However, so long as the immiscibility is maintained, there appears to be no reason why the density of the indicator liquid 30 might not be the same or even less than the density of the liquid 11.
It will be observed that the larger tube 20 has an inside cross-sectional area which is indicated by an arrow across the diameter. It has reference No. 31 applied thereto. This area is greater than the inside cross-sectional area of the legs 16 and 17 of the U-tube. The latter area is indicated by the dimension d2 which has arrows with reference numeral 32 applied thereto (in FIG. 1).
It has been discovered that the changes in the level or height of the surface 12 of the body of liquid 11, will produce a differential in the levels of the indicator liquid 30 in U-tube 15.- And, the ratio of the change in level of surface 12, to the differential in the levels of the indicator liquid - - : . . ~

108384~3 30 in the legs 16 and 17, is in accordance with the ratio of the inside cross-sectional area of the legs 16 and 17 to the inside cross-sectional area of the tube 20. Consequently, there is a substantial amplification of the change in the level of the surface 12. Further- ;
more, with the density of the indicator liquid 30 being the same or close to the same density as the body of liquid 11, the changes in levels are sub-stantially all on account of the ratios of cross-sectional areas. As will appear below, this is because the volumne of liquid which must move during the establish-ment of a new balance, is the major factor in the change in liquid levels.
The foregoing relationship was discovered to exist while employing an indicator liquid that was made of a mixture of water and methanol, and with the body of liquid having its surface level measured being gasoline.
An explanation of why the amplification of liquid level ch~nge takes place, may be made clear by reference to the schematic diagrams of FIGS. 1 and 2.
Thus, with relation to the various liquid level changes and the sizes of the J-tube structure, consider the following. The procedure involves first the immersion of the J-tube structure, i.e. U-tube 15 and the larger tube 20 connected to leg 1~ thereof, all into the body of liquid 11. The immersion is done with valves 23 and 24 both open. Then the liquids in both legs of the J-tube system are permitted to equalize and the surfaces of the indicator liquid 30 will be at the same level. This level is indicated in the diagrams ~083848 by a dashed line 35.
Next, when it is desired to commence a fluid level measuring time period, the valve 24 will be closed so as to isolate the liquid inside of tube 20 and the connecting leg 17 of the U-tube 15. ~hereafter, following some predetermined period of time it may be found that the fluid level 12 of the body of liquid 11 has fallen to the position of the surface 12 which i8 indicated in FIG. 2. The difference between these levels is indicated by the symbol in FIG. 2.
The change a h in the level of surface 12 will cause a difference in pressure on the indicator-liquid 30 in the two legs of the U-tube 15, because of the fact that the liquids in tube 20 and the leg 17 are isolated from the body of liquid 11. Consequently, the indicator liquid 30 will be pushed down in the leg 17 of the U-tube 15, and a corresponding change must take place in raising the liquid level of the indicator liquid 30 in leg 16. Since the liquid 11 and indicator liquid 30 are immiscible, all of the liquid which was isolated from the body of liquid 11 in the long leg of the J-tube structure remains isolated and there must be an equal volume of the liquid displaced on the leg 16 side of the U-tube as the volume which has moved down from the tube 20 into the leg 17 of the U-tube, as the change in liquid levels took place.
Because of the foregoing conditions, the changes in fluid levels will be directly in proportion to the ratios of the cross-sectional areas of the fluid columns involved or to the square of the diameter ,: .

~083848 of each of the fluid columns.
The foregoing relationship may be proved mathematically in the following manner, with reference to the dimensions indicated on EIGS. 1 and 2, and assuming that the tube 20 and the legs 16 and 17 are circular in cross-section. Thus, it will be observed that:
4 h = h6 + h7 (1) ~hen, h6 can be obtained in terms of h2 from the fact that the volumne of liquid that has left the large diameter tube 20 is equal to the volume of liquid that has entered the small diameter leg 17. The latter is the distance h2 divided by two. This may be expressed by the following equations:
7r/4 dl2 h6 = ~r/4 d2 h2/2 (2) which may be solved for h6 and will result in the equation:
h6 = (d2/dl )2 h2/2 Then, h7 can be obtained in terms of h2 from the fact that:
h7 = h4 - (h3 + h2 ) = (h4 - h3 ) - h2 (4) It will be noted that the quantity h4 - h3 can be expressed in terms of h2 as follows, and involving the 1uid pressures: the pressure at the depth h3 may be expressed as h3 = h3 pg (5) wherein pg = the density of the liquid 11.
Then considering the other side (isolated portions) of the U-tube columns, the pressure at depth h4 which is due to the isolate head of liquid 11 above the indicator fluid level in the leg 17 of the U-tube . ~ :
., --. . . .~ , . : - ., .. ~ . : . , .

may be expressed as h4 = h4 p (6) And, since we ha~e a static balance, we can express the dif~erence in pressure between the depths h3 and h4 as being equal to the expression h2 Pw ~ wherein Pw = the density of the indicator liquid 30. Thus:
h4 pg - h3 pg = h2 Pw or, h4 - h3 = h2 Pw/pg Next, it is clear that:
h7 = (h4 - h3 ) - h2 (8) and substituting in that equation for the quantity h4 - h3 from the foregoing equation (7) we have h7 = h2 Pw/pg h2 h2 (Pw/Pg~ 1 ) (9 ) Now, since ~h = h6 + h7 , it may be rewritten by substitutions from equations (3) and (9) to be h = (d2/dl) h2/2 + h2(pw/pg 1) = h2 [1/2 (d2/dl) + Pw/Pg (10) From this it can be seen that if the densities, i.e.

Pw and pg are substantially equal, the difference in liquid levels from the beginning to the end of a measurement time period, i.e. h, will be reflected at ~he levels of the indicator-liquid 30 surfaces as one half of h times the ratio of the inside diameter d2 divided by dl , squared. Or:
~ h = h2/2 (d2/dl)2 (11) In other words, the change in level 12 of the liquid 11, will be small compared to the change in the level of the surfaces of the indicator liquid columns of liquid 30, in the U-tube legs 16 and 17. Thus, the latter is an amplification of the former, in accordance .

with a ratio that is mostly related to the inside cross-sectional areas of the larger and smaller columns when the density of the indicator liquid 30 is nearly the same as that of the body of liquid ll.
An instrument constructed in accordance with the invention, which may be employed for measuring change in the surface level of a fluid, is illustrated in FIG. 3. This includes the various elements which will be descri~ed in more detail below. The instrument will be immersed in a body of liquid 40 which is to have any change in surface level thereof measured.
There is a relatively small inside diameter U-tube 41 which has a pair of legs 44 and 45. The U-tube 41 contains a predetermined quantity of an indicator liquid 48 therein, which is immiscible with the liquid 40. In-addition, the indicator liquid 48 will preferrably have a color added thereto for aiding observation o~ the surface levels thereof.
An instrument that was tested, employed as the indicator liquid 48, a mixture of methanol and 5% water, which was dyed slightly to make it distinguishable visually from the body of liquid 40 which was gasoline.
Of course, the legs 44 and 45 of the U-tube portion 41, were transparent.
In the instrument illustrated in FIG. 3, a relatively large inside diameter tube 51 is connected to the top of the leg 45 of the U-tube 41. The large ~ - .
tube 51 is long enough to extend from above the maximum surface level of the body of liquid 40, during a measure-ment, to below the minimum surface level thereof. ~;

108384~

There is a three-way valve 52 that is schematically indicated in FIGS. 4 and 5. This valve 52 has three ports 56, 57 and 58. These ports are connected to the legs 44 and 45 of the U-tube 41, as well as to the body of liquid 40, in the manner indicated by the schematic diagrams of FIGS. 4 and 5.
~ alve 52 is located on the instrument so that the connections to U-tube legs 44 and 45 are above the maximum height of the surfaces of indicator liquid 48 therein. Also, it has at least three different positions, two of which are indicated by the schematic showings in FIGS. 4 and 5. One position is like that shown in FIG. 4 which connects the port 57 directly wilh common internal passages that are indicated schematically by three arrows 60. Consequently, the port 57 is connected to port 58 and port 56 together.
Thus, in this position the leg 45 of the J-tube instrument is connected to the leg 44 and to body of liquid 40.
Therefore the indicator liquid 48 will stabilize its surfaces at equal levels, so that the indicator liquid 48 has equal height columns in the legs 44 and 45.
It will be observed that the instrument also has a reservoir 61 that is connected into the leg 44 of the U-tube 41. This reservoir 61 is provided in order to contain any overflow of the indicator liquid 48, in case the response becomes too great.
Also, there is an elongated support member 62 which, has the tubes of the J-tube instrument attached in any convenient manner. It is for handling the instrument and to support it when a measurement is ~ -.

being taken.
There is an elongated handle 65 which connects to the three-way valve 52 and extends upward sufficiently to rise above the surface of the liquid 40 for manual manipulation of the valve.
It will be appreciated that a measurement procedure to detect a change in surface level of the liquid 40 will involve an instrument like that described above, and a step of immersing the J-tube instrument into the body of liquid 40. Then the instrument will be hela at a fixed position to allow equalization of the columns of indicator - liquid 48 in the legs 44 and 45. This equalization will take place with the valve 52 in the position indicated by FIG. 4.
At the commencement of a measuring time period the valve 52 will be shifted to the position indicated in FIG. 5, and consequently the liquid column on the long side (tube 51 and leg 45) of the J-tube instrument will be isolated from the liquid 40. After :
a predetermined period of time, any change in the level of the body of liquid 40 will be reflected by the amplified change in levels of the surfaces of indicator ~ .
liquid 48, in the manner indicated above. This change will take place while the valve 52 is in a position .
shown in FIG. 5.
Then, in order to take a reading of the indicator - fluid surface levels in the U-tube legs 44 and 45,.the situation may be held at then existing conditions by shifting the valve 52 to a position (not illustrated) which will close both ports 57 and 58.

.

This holds the liquid levels in legs 44 and 45 at the positions they had reached. Then the instrument may be raised bodily up out of the liquid 40 in order to take a reading of the difference in levels of the surfaces of indicator liquid 48.
It will be appreciated by any one skilled in the art that the same principles apply to a structure which may be employed as a sensitive manometer. ~hus, with reference to FIG. 6 there is illustrated a U-tube structure 70. The bottom portion contains an indicator-liquid 73 which may be like the indicator liquid of the above described modifications.
The indicator liquid 73 is immiscible with the liquid columns 74 and 75 that are located above the upper surfaces of the indicator liquid 73. These liquid columns 74 and 75 are continued in small size portions 78 and 79 respectively, above the indicator liquid 73 in the U-tube 70. It will be appreciated from the description of the foregoing J-tube modification that the legs 78 and 79 and the bottom of the U-tube have a predetermined relatively small inside cross-sectional area.
Connected to the tops of the legs 78 and 79 there are a pair of extensions 80 and 81 respectively, and these are enlarged so that the inside cross-sectional areas are greater than that of the legs 78 and 79.
The liquid column 74 and 75 will be filled to an equal level at the surfaces thereof, when the indicator liquid 73 stands in balance with its upper surfaces at the same level. In order to ensure stability under such conditions, it is preferable - - ''' ` ~:
: .
.

10838~3 for the indicator liquid 73 to have a slightly greater density than the liquid in the ~olumns 74 and 75.
Since the instrument 70 is intended for use as a manometer, it will be usual to have the extensions 80 and 81 constructed with a reduction of the diametric size thereof at the top of each. This provides tubes or passages for application of gas pressures that are to be measured. It will be appreciated and can be shown, that in the manometer instrument according to FIG. 6, a difference in pressure applied to the two liquid columns 74 and 75, will produce an amplified difference in the surface heights of indicator liquid surfaces 73.
The same principles as were explained above in connection with the other modifications of the invention are , applicable. Such amplification will be at a ratio whlch is close to that of the cross-sectional areas of the larger and smaller portions of each of the two liquid columns making up the U-tube 70. This, of course, assumes that the density of the indicator liquid 73 is quite close to the same density as the liquid in columns 74 and 75 there above.
While particular embodiments of the invention have been described above in considerable detail in accordance with the applicable statutes this is not to be taken as in any way limiting the invention but _ rely as bei~g desor1ptive thereof.

. ~,' ~.

-~ - 16 -,,~, . .. ... .. .. ~ ~ , . .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Sensitive J-tube device for measuring change in the surface level of a fluid, comprising a relatively small inside diamater U-tube containing a predetermined quantity of a colored indicator-fluid that is immiscible with said fluid the surface level of which is to be measured, said indicator-fluid having a density approximately the same but heavier than said fluid being measured, a relatively large inside diameter tube connected to one leg of said U-tube and forming the taller leg of said J-tube, said large tube being long enough to extend from above the maximum surface level of said fluid being measured to below the minimum surface level thereof, a three-way valve having three ports for connection to each of the legs of said U-tube and to the body of said fluid being measured, means for connecting said U-tube leg ports to said U-tube at a location that is above the maximum height of said indicator-fluid therein, said three-way valve having at least three positions one for connecting all three of said ports together and one for connecting only the shorter leg of said J-tube to the body of said fluid being measured and one for closing said U-tube leg ports, a reservoir connected into the shorter leg of said J-tube at a location above the maximum expected height of said indicator-fluid in order to contain any overflow without loss of indicator-fluid.
an elongated support for holding said J-tube device in a fixed position when a measurement is being taken, and an elongated handle for actuating said three-way valve from a location above the surface of said fluid being measured.

2. Method of measuring a change in surface level of a fluid, comprising the steps of taking a relatively small inside cross-sectional area U-tube having an indicator-fluid therein, connecting a larger inside cross-sectional area tube to one leg of said U-tube, immersing said tubes into said fluid with said larger area tube extending above said surface level, connecting both legs of said U-tube with said fluid at a location on said tubes which is above the maximum height of said indicator-fluid therein, said connection being made while said tubes are immersed, disconnecting said one leg from said fluid to isolate said leg and said larger area tube from said fluid at the beginning of a measuring time period, and determining the difference in height of the tops of said indicator-fluid levels in said U-tube at the end of said measuring time period whereby the change in surface level of said fluid is magnified.

3. Method of magnifying the measurement of a change in surface level of a first fluid by employing a U-tube containing a second fluid which is immiscible with said first fluid, comprising the steps of connecting a relatively large cross-section area tube to one leg of said U-tube at a location that is higher than a predetermined maximum level of rise of said second fluid in one of the legs of said U-tube, connecting both legs of said U-tube into communication with said first fluid at a location above said maximum level prior to the commencement of a measurement time period, immersing said U-tube and said large area tube in said first fluid to equalize the level of said first fluid inside of said large area tube with that outside and equalize the levels of said second fluid in said U-tube legs, isolating said one leg and the large cross-section area tube from said first fluid at said commencement of said measurement time period, and determining the difference in fluid levels of said second fluid in said U-tube legs at the termina-tion of said measurement time period.

4. Sensitive J-tube instrument for measuring change in the surface level of a fluid in an underground tank or the like, comprising a U-tube portion containing an indicator fluid therein, said indicator fluid being immiscible with said fluid the change in surface level of which is to be measured, said U-tube portion having a predetermined inside cross-section area, one leg of said U-tube portion being connec-ted to a larger inside cross-sectional area tube to form the taller leg of said J-tube, said larger inside cross-sectional area tube having a length greater than the maximum differ-ence in surface levels of said fluid being meas-ured between the maximum and minimum surface levels during a measurement thereof, and valve means for connecting each of said U-tube portion legs with said fluid being measured and for isolating said taller leg, said valve means being connected above the tops of said indicator fluid.

5. Measuring instrument according to Claim 4, wherein said indicator fluid has a density approximately the same but heavier than said fluid to be meas-ured.

6. Measuring instrument according to Claim 5, wherein said indicator fluid includes a coloring agent for visually distinguishing from said fluid to be measured.

7. Measuring instrument according to Claim 6, wherein said valve means comprises a three-way valve.

8. Measuring instrument according to Claim 7, further comprising a reservoir connected to the other leg of said U-tube portion at a location above the maximum expected height of said indicator fluid in order to contain any overflow without loss of indicator fluid.

9. Measuring instrument according to Claim 8, further comprising an elongated support for suspending said instru-ment in said fluid to be measured.

10. Measuring instrument according to Claim 9, further comprising elongated means for actuating said three-way valve from a location above the surface of said fluid to be measured.